utf8::downgrade( my $empty_dn = ''          );  # 0
utf8::upgrade(   my $empty_up = ''          );  # 1
utf8::downgrade( my $ascii_dn = 'a'         );  # 0
utf8::upgrade(   my $ascii_up = 'a'         );  # 1
utf8::downgrade( my $hibit_dn = chr(0xC9)   );  # 0
utf8::upgrade(   my $hibit_up = chr(0xC9)   );  # 1
utf8::upgrade(   my $wide_up  = chr(0x2660) );  # 1

for (
    $empty_dn, $empty_up,
    $ascii_dn, $ascii_up,
    $hibit_dn, $hibit_up,
    $wide_up,
) {
   print is_utf8($_)?1:0, "\n";
}